Abstract: In a rotor -bearing system geared, there are many factors that affect dynamic characteristic of the system. The influence of the type of bearing, the gap ratio, and the ratio between width and diameter on the instability threshold speed, the whirl ratio, the critical speed and its distribution, and damping characteristic of the system is only researched. It is found that, in the system, there is a weak portion that affects the stability of the system. When the type of bearing, the gap ratio, and the ratio between width and diameter changes, the instability threshold speed and the logarithm decrement of the system will also change. It is put forward that least logarithm decrement can be used to judge the ability to stride over the critical speed of the system, the which ratio can be used to decide which rotor firstly becomes unstable.

Key words: Bearing Parameters, Critical speed, Damping, Stability, Whirl ratio